The invention is directed to improving the core loss of cube-on-edge grain oriented electrical steels. In such electrical steels, the body-centered cubes making up the grains or crystals are oriented in a cube-on-edge position, designated (110) [001] in accordance with Miller's Indices.
Cube-on-edge oriented silicon steels are well known in the art and are commonly used in the manufacture of cores for transformers and the like. Cube-on-edge electrical steels are produced by a number of routings typically involving one or more operations of cold rolling and one or more operations of annealing, so as to obtain a cold-rolled strip having a commercial standard thickness. After the cold rolling is completed, the strip may be subjected to a decarburizing anneal and coated with an annealing separator. Thereafter, the sheet is subjected to a high temperature final anneal at a temperature of about 1200.degree. C. As used herein and in the claims, the term "high temperature final anneal" refers to that anneal during which the cube-on-edge texture is produced as the result of secondary grain growth. The now-oriented electrical steel has its easiest axis of magnetization in the rolling direction of the sheet so that it is advantageously used in the manufacture of magnetic cores for transformers and the like.
Various specific routings devised in recent years by prior art workers have resulted in cube-on-edge grain oriented silicon steels having markedly improved magnetic characteristics. As a consequence, such electrical steels are now considered to fall into two basic categories.
The first category is generally referred to as regular grain oriented silicon steel and is made by routings which normally produce a permeability at 796 A/m of less than 1870 with a core loss at 1.7 T and 60 Hz of greater than 0.700 W/lb when the strip thickness is about 0.295 mm.
The second category is generally referred to as high permeability grain oriented silicon steel and is made by routings which normally produce a permeability at 796 A/m of greater than 1870 with a core loss less than 0.700 W/lb (at 1.7 T and 60 Hz) when the strip thickness is about 0.295 mm.
U.S. Pat. No. 3,764,406 is typical of those which set forth routings for regular grain oriented silicon steel. For regular grain oriented silicon steel, a typical melt composition by weight percent may be stated as follows:
C: less than 0.085% PA0 Si: 2%-4% PA0 S and/or Se: 0.015%-0.07% PA0 Mn: 0.02%-0.2% PA0 Si: 2%-4% PA0 C: &lt;0.085% PA0 Al (acid soluble): 0.01%-0.065% PA0 N: 0.003%-0.010% PA0 Mn: 0.03%-0.2% PA0 S: 0.015%-0.07%
The balance is iron and those impurities incident to the mode of manufacture.
In a typical but non-limiting routing for regular grain oriented silicon steel, the melt may be cast into ingots and reduced to slabs, continuously cast in slab form or cast directly into coils. The ingots or slabs may be reheated to a temperature of about 1400.degree. C. and hot rolled to hot band thickness. The hot rolling step may be accomplished without reheating, if the ingot or slab is at the required rolling temperature. The hot band is annealed at a temperature of about 980.degree. C. and pickled. Thereafter, the silicon steel may be cold rolled in one or more stages to final gauge and decarburized at a temperature of about 815.degree. C. for a time of about 3 minutes in a wet hydrogen atmosphere with a dew point of about 60.degree. C. The decarburized silicon steel is thereafter provided with an annealing separator, such as a coating of magnesia, and is subjected to a final high temperature box anneal in an atmosphere such as dry hydrogen at a temperature of about 1200.degree. C. to achieve the desired final orientation and magnetic characteristics.
U.S. Pat. Nos. 3,287,183; 3,636,579; 3,873,381; and 3,932,234 are typical of those teaching routings for high-permeability grain oriented silicon steel. A nonlimiting exemplary melt composition for such a silicon steel may be set forth as follows in weight percent:
The above list includes only the primary constituents; the melt may also contain minor amounts of copper, phosphorus, oxygen and those impurities incident to the mode of manufacture.
In an exemplary, but non-limiting, routing for such high-permeability grain oriented silicon steel, the steps through hot rolling to hot band thickness can be the same as those set forth with respect to regular grain oriented silicon steel. After hot rolling, the steel band is continuously annealed at a temperature of from about 850.degree. C. to about 1200.degree. C. for from about 30 seconds to about 60 minutes in an atmosphere of combusted gas, nitrogen, air or inert gas. The strip is thereafter subjected to a slow cooling to a temperature of from about 850.degree. C. to about 980.degree. C., followed by quenching to ambient temperature. After descaling and pickling, the steel is cold rolled in one or more stages to final gauge, the final cold reduction being from about 65% to about 95%. Thereafter, the steel is continously decarburized in wet hydrogen at a temperature of about 830.degree. C. for about 3 minutes at a due point of about 60.degree. C. The decarburized silicon steel is provided with an annealing separator such as magnesia and is subjected to a final box anneal in an atmosphere of hydrogen at a temperature of about 1200.degree. C.
It is common practice, with respect to both types of grain oriented silicon steels, to provide an insulative coating having a high dielectric strength on the grain oriented silicon steel (in lieu of, or in addition to, a mill glass). The coating is subjected to a continuous anneal at a temperature of about 815.degree. C. for about 3 minutes in order to thermally flatten the steel strip and to cure the insulative coating. Exemplary applied insulative coatings are taught in U.S. Pat. Nos. 3,948,786; 3,996,073; and 3,856,568.
The teachings of the present invention are applicable to both types of grain oriented electrical steels.
The pressure of increasing power costs has demanded that the materials used for transformer cores and the like have the lowest core loss possible. Prior art workers have long addressed this problem and have devised a number of methods to reduce core loss of grain oriented electrical steels.
For example, it is well known that core loss of oriented electrical steels can be decreased by increased volume resistivity, reduced final thickness of the electrical steel, improved orientation of the secondary grains, and by decreased size of the secondary grains. The process of secondary grain growth is regulated by the presence of a dispersed phase comprising such elements as manganese, sulphur, selenium, aluminum, nitrogen, boron, tungsten and molybdenum (and combinations thereof) as well as the grain structure (e.g. primary grain size and crystal texture) of the electrical steel prior to the final high temperature anneal. All of these metallurgical variables must, however, be kept within prescribed limits to attain the optimum core loss in the finished grain oriented electrical steel. Maintaining this metallurgical balance has inhibited the development of materials with core losses closer to the theoretical limits.
Prior art workers have also turned their attention to methods of regulating the size of the secondary grains through the use of local deformation. Local deformation by bending prior to the final anneal so as to regulate the size of the cube-on-edge grains has been taught. This method, however, is difficult to employ in practice because of the difficulty of the bending operation.
U.S. Pat. No. 3,990,923 teaches a number of methods of local working of the electrical steel surface by local plastic working employing shot peening or rolling with grooved rolls. This reference also teaches local thermal working employing an electron beam or laser irradiation. Both the mechanical and thermal working techniques taught in this reference produce finer primary grains in the worked bands immediately after the treatment. Such local working methods serve to increase the amount of stored energy in the locally worked bands, and must be limited to a depth of about 70 .mu.m (0.04 mils) in order to regulate secondary grain growth during the final high temperature anneal. Again, the techniques taught in this reference are difficult to employ in practice, particularly at line speeds.
The present invention is based on the discovery that if the cube-on-edge grain oriented electrical steel is subjected to local annealing after at least one stage of cold rolling and before the final high temperature anneal, bands of enlarged primary grains are produced which regulate the growth of the secondary cube-on-edge grains in the intermediate unannealed areas of the electrical steel during the final high temperature anneal. This procedure reduces the amount of stored energy within the locally annealed bands which results in an enlargement of the primary grains within the locally annealed bands and throughout the thickness of the strip. The enlarged primary grains in the annealed bands are, themselves, ultimately consumed by the secondary grains. As a result, a cube-on-edge grain oriented electrical steel with smaller secondary grains and reduced core loss is produced.
The local annealing treatment of the present invention is rapid, and an annealed band across the full strip width can be formed in less than one second. Therefore, it can be readily inserted in the pre-existing process technology and appropriately adapted to line speeds. The local annealing step is easy to regulate since the annealing is controlled by such factors as heat input to the annealed band, time and percent reduction in the cold rolling prior to the local annealing treatment. The resulting smaller secondary grain size and accompanying reduced core loss values are stable and will be unaffected by subsequent stress relief annealing or the like.